This invention relates to airborne ion balance and concentration sensing and monitoring.
Static electric charge accumulation can cause severe problems in variety of manufacturing processes and industrial operations. One of the methods of coping with electric charge build-up is to create a volume region of highly ionized air in the immediate vicinity of objects that are to be protected. If these objects become electrically charged, they attract air ions of the opposite polarity. This, in turn, leads to electric charge neutralization. Ionized air, containing ions of positive and negative polarities, is usually provided by an air ionizing system. Such system has to be periodically verified in order to assure its proper functioning, and in critical environments a continuous ionizer system monitoring may be necessary.
A typical evaluation of an air (gas) ionizer consists of three parts. The first two tests measure the ionizer's capability of delivering positive and negative ions at the desired production rate level, so the protected objects can be neutralized within certain time limits. According to the ANSI ESD STM 3.1-2000 standard (EOS/ESD Association Standard for Protection of Electronic Discharge Susceptible Items - Ionization, ESD Association 2000), this ability is determined by the time required to discharge a charged plate, having a specified capacitance relative to ground, between specified voltage levels. To accomplish this, the plate is first pre-charged to an initial voltage level and then allowed to discharge to typically 10% of the initial test voltage. The time required for the discharge is recorded for both polarities of the initial voltage. These two measurements are called discharge time tests.
The third part of ionizer evaluation is a voltage offset measurement. A test plate is first shorted to the earth ground to remove any residual charge. The plate is then disconnected from the ground and allowed to float. The voltage measured on the plate after plate voltage stabilization is a result of the net charge collected from the airborne ions impinging on the plate. The stabilized plate voltage value also indicates the voltage level to which objects placed into the ion field that are of similar size and geometry as the test plate will be driven to by the ion field.
Evaluation of the air (gas) ionizer system using the above described prior art method is time consuming and not robust enough for application in continuous ionizer monitoring. Reference may be made to U.S. Pat. No. 6,130,815 issued Oct. 10, 2000 entitled “Apparatus and Method for Monitoring of Air Ionization”, U.S. Pat. No. 5,506,507 issued Apr. 9, 1996 entitled “Apparatus for Measuring Ions in a Clean Room Gas Flow Using a Spherical Electrode” and U.S. Pat. No. 6,433,522 issued Aug. 13, 2002 entitled “Floating Plate Monitor”, the disclosures of all three of the foregoing patents being incorporated herein by reference. Proper monitoring requires simultaneous measurement of ion balance (voltage-offset test) and of ion production rate for both ion polarities (discharge time test). Existing charge plates and charged plate monitors are not capable of such simultaneous test. Usually, these instruments monitor the voltage offset only, and in certain systems additionally provide a feedback signal to the ionizer. This feedback information about ion imbalance can be used to control the ionizer system; however it does not indicate whether the ionizer produces amounts of ions sufficient enough to discharge the protected object within the desired time. It informs only about the ratio of positive vs. negative ions reaching the plate by indicating the voltage offset.